The present invention relates to a multi-stage hydraulic cylinder for providing multiple stages of displacement with different cylinder forces, comprising a cylinder housing, a first stage piston slidably received in said cylinder housing to be displaced over a first stage of displacement, a second stage piston coupled to said first stage piston to be displaced, together with said first stage piston, over said first stage of displacement and to be displaced relative to said first stage piston over a second stage of displacement, said first stage piston and said second stage piston having pressure surface areas different from each other to provide for said different cylinder forces, and further comprising a cushion arrangement for smoothening stage changes.
Certain applications of hydraulic cylinders require different displacement forces at different stages of displacement. For example, in large mining trucks, the dumping body for collecting and transporting the mining material is tilted about a horizontal axis to dump the material, said tilting movement requiring a large driving force at the initial stage of tilting, whereas the required driving force is considerably decreased when the dumping body reaches a more upright position with the center of gravity being closer to the tilting axis and a part of the material already having been dumped.
In such hydraulic systems where one or more fixed or variable displacement pumps may be used for operating one or more multi-stage hydraulic cylinders, there exists mechanical contact at the time of stage change. In applications where the force required from the cylinder decreases dramatically as extended length increases, it is desirable to have considerably smaller second stage than first stage, resulting in a large area ratio. However, such large area ratio causes a shock when the first stage piston reaches its mechanical stop. In certain applications of such multi-stage hydraulic cylinders, basically the same pressure is supplied to both the first stage piston and the second stage piston so due to the large area ratio of such pistons a rather hard shock is caused when the first stage piston reaches the end of its displacement path and hits against the mechanical stop.
In order to reduce such shocks, a cushioning arrangement may be used to decelerate the piston at the end of its displacement path before the piston hits said mechanical stop, thereby reducing the velocity of the piston when hitting against the mechanical stop. Various solutions for such cushioning arrangements have been proposed. For example, document U.S. Pat. No. 4,397,218 discloses a cushioning device for decelerating and stopping the piston in a hydraulic cylinder by restricting and throttling the fluid flow from the cylinder. More particularly, the flow of fluid is reduced as the end of stroke in the cylinder is reached. A plunger is slidably received in a flow path, said plunger being pushed deeper into said flow path by the piston approaching its end position, wherein the flow path gets further restricted as the plunger is inserted deeper. However, the downside to this, especially in a fixed displacement system, is any fluid that does not do work on the cylinder is required to flow over a relief valve creating unnecessary heat.
Furthermore, document U.S. Pat. No. 7,104,054 discloses a way to electronically control the speed as a piston reaches the end of its stroke. The hydraulic cylinder system includes a sensor configured to generate a signal indicative of hydraulic cylinder position and a valve coupled to the cylinder to control flow rate of hydraulic fluid to and from the cylinder so that an electronic controller that is coupled to the sensor may command valve opening and closing in response to the hydraulic cylinder position. Such electronic sensing and commanding system is quite expensive, increases complexity and is prone to damages and functional errors under rough environmental conditions.
Furthermore, document US 2006/0151269 discloses a cushioning device that completely prevents the piston from colliding with the end boss. At the end of the piston stroke, a portion of the hydraulic oil to be discharged from the pressure chamber is trapped and may be discharged via a throttle clearance only. In addition, an elastic cushion element made of rubber is positioned between the end boss and the piston to prevent the piston from directly hitting onto the end boss. Such elastic cushion, however, is not advantageous in certain applications due to multiple reasons. In particular, the space required to have an elastic cushion element is not available in certain cylinders such as multi-stage cylinders where a plurality of pistons are to be received within the cylinder housing. Also the forces present in cylinders for large applications such as mining trucks or other large construction equipment prohibit effective use of an elastic cushion element made of rubber. Furthermore, the durability of an elastic cushion element is rather limited in applications where the cylinder is operated at high frequencies and high loads.